INTRODUCTION

An uninterruptible power supply (UPS) is an electrical system capable of supplying high quality electrical power without interruptions. The mains electrical supply is connected to the input of the UPS and the output is connected to the customer electrical load. Within the UPS system there are power supply storage systems such as batteries and flywheels which are capable of providing a high quality electrical supply. A UPS not only provides protection against all types of power supply failure, but can also filter a vast range of disturbances found in the mains supply, thus providing more sensitive loads with a clean power supply. A UPS can provide power to a critical load while an alternative supply, such as a stand-by generator, is brought on-line. In this case, the UPS may only need to support the critical load for a short period, perhaps five to 10 minutes. However, a UPS can also be designed to support the critical load for much longer - possibly up to one hour. In this case, significant extra storage capacity will be needed. The growth in voice and data communications and online processing, such as mobile phones, email and online banking, has made having a secure power supply an increasingly important issue. A UPS is now common in computer rooms, data centres and server areas, and plays a significant part in maximising the availability of systems. It is important that UPS systems not only avoid reducing the quality of the electrical supply, but also smooth out any peaks or spikes in the power supply which could damage the equipment.

The switchover time can be as long as 25 milliseconds depending on the amount of time it takes the standby UPS to detect the lost utility voltage. The SPS then mechanically switches the connected equipment on to its DC-AC inverter output. providing surge protection and battery backup. When the incoming voltage falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry. The UPS will be designed to power certain equipment. without any objectionable dip or brownout to that device. which is powered from an internal storage battery. The protected equipment is normally connected directly to incoming utility power. Capacity expansion: Usually not available
The offline / standby UPS (SPS) offers only the most basic features. such as a personal computer.Offline / standby
Offline / standby UPS. Typical protection time: 0–20 minutes.
Benefits:    Low cost High efficiency Compact size
Limitations:  Uses battery during brownouts
.

The battery can still be charged while "bucking" an overvoltage. and then switch to battery if the voltage goes much higher or lower than that range. Typical protection time: 5–30 minutes. thereby increasing or decreasing the magnetic field and the output voltage of the transformer. the transformer output is too low to charge the batteries. and a second one to convert the line voltage to a slightly [clarification needed] higher battery charging voltage (such as a multiple of 14 V) . Further. This is a special type of transformer that can add or subtract powered coils of wire. while the load is connected to the other. changing the autotransformer tap can cause a very brief output power disruption. Autotransformers can be engineered to cover a wide range of varying input voltages. equipped with a power-loss alarm to "chirp" for a moment. thus using the main transformer's primary windings as an autotransformer. but with the addition of a multi-tap variable-voltage autotransformer. Capacity expansion: Several hours
The line-interactive UPS is similar in operation to a standby UPS.
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which may cause UPSs
. but this requires more taps and increases complexity. Depending on the design. This type of UPS is able to tolerate continuous undervoltage brownouts and overvoltage surges without consuming the limited reserve battery power. The main 50/60 Hz transformer used to convert between line voltage and battery voltage needs to provide two slightly different turns ratios: one to convert the battery output voltage (typically a multiple of 12 V) to line voltage. and expense of the UPS. To gain the buck/boost feature. but while "boosting" an undervoltage. It is common for the autotransformer to cover a range only from about 90 V to 140 V for 120 V power. It instead compensates by automatically selecting different power taps on the autotransformer. This has become popular even in the cheapest UPSs because it takes advantage of components already included.[edit]Line-interactive
Line-interactive UPS. all that is required is two separate switches so that the AC input can be connected to one of the two primary taps. it is easier to do the switching on the line-voltage side of the transformer because of the lower currents on that side.

the rectifier resumes carrying most of the load and begins charging the batteries. due to it having a much greater current AC-to-DC batterycharger/rectifier. so that no power transfer switches are necessary. However it typically costs much more. When power is restored. the UPS will need less current. For example to power a 1000-watt device at 120 volts. the batteries are always connected to the inverter. supplying 500 watts or less. even when powered from normal AC current. though the charging current may be limited to prevent the high-power rectifier from overheating the batteries and boiling off the electrolyte. the UPS will draw 10 amperes to compensate. the rectifier simply drops out of the circuit and the batteries keep the power steady and unchanged. In an online UPS. When power loss occurs. Although once previously reserved for very large installations of 10 kW or more. so that in an overvoltage condition. and with the rectifier and inverter designed to run continuously with improved cooling systems. Benefits: High reliability High efficiency Good voltage conditioning Limitations: Can not be use over a range of 5kVA [edit]Online / Double-conversion The online UPS is ideal for environments where electrical isolation is necessary or for equipment that is very sensitive to power fluctuations. The online UPS may be necessary when the power environment is "noisy". outages and other anomalies are frequent.
. advances in technology have now permitted it to be available as a common consumer device.In low-voltage conditions the UPS will use more current than normal so it may need a higher current circuit than a normal device. It is called a double-conversion UPS due to the rectifier directly driving the inverter. The main advantage to the on-line UPS is its ability to provide an electrical firewall between the incoming utility power and sensitive electronic equipment.33 amperes. or when operation from an extended-run backup generator is necessary. when protection of sensitive IT equipment loads is required. when utility power sags. If a brownout occurs and the voltage drops to 100 volts. but its total cost of ownership is generally lower due to longer battery life. The basic technology of the online UPS is the same as in a standby or line-interactive UPS. This also works in reverse. the UPS will draw 8. The initial cost of the online UPS may be slightly higher.

[edit]Hybrid topology / double conversion on demand These hybrid designs do not have an official designation. When the power conditions fluctuate outside of the predefined windows. This allows the UPS to achieve very high efficiency ratings. HP R12000. can filter out line noise and control frequency. the UPS switches to online/double conversion operation. Examples of this hybrid/double conversion on demand UPS design are the HP R8000. although one name used by HP and Eaton is double conversion on demand.
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In double conversion mode the UPS can adjust for voltage
variations without having to use battery power.
.
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This style of UPS is targeted towards high efficiency applications
while still maintaining the features and protection level offered by double conversion. HP RP12000/3 and the Eaton BladeUPS. A hybrid (double conversion on demand) UPS operates as an off-line/standby UPS when power conditions are within a certain preset window.Benefits: Excellent voltage conditioning Limitations: Lower efficiency Expensive under 5kVA.

Benefits: Excellent voltage conditioning High reliability Limitations: Low efficiency Limited application because low unstable in efficiency and instability issues. These units are still mainly used in some industrial settings (oil and gas. and heavy industry markets) due to the robust nature of the UPS. Many ferro-resonant UPSs are 82–88% efficient (AC/DC-AC) and offer excellent isolation. Many ferro-resonant UPSs utilizing controlled ferro technology may not interact with power-factor-correcting equipment. one for ordinary mains power. however. petrochemical. utility.
. the second for rectified battery power. chemical. and the third for output AC power to the load. The transformer has three windings. they are online with the exception that a ferro-resonant transformer is used to filter the output.Ferro-resonant
Typical protection time:
5–15 minutes
Capacity expansion:
Several Hours
Ferro-resonant units operate in the same way as a standby UPS unit. This once was the dominant type of UPS and is limited to around the 150 kVA range. This transformer is designed to hold energy long enough to cover the time between switching from line power to battery power and effectively eliminates the transfer time.

Rather than converting AC to DC to charge batteries. but that is not generally a significant problem. but is subject to the more complex electrical code rules for safe containment of high voltages.DC power
Typical protection time:
Several hours
Capacity expansion:
Yes
A UPS designed for powering DC equipment is very similar to an online UPS. so it is certain to be able to handle the unbalance when operated from DC in the 230 V position.
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Most switched-mode power supply (SMPS) power supplies for PCs can handle 325 V DC (230 V mains voltage × √2) directly. to supply the same amount of power. and often the powered device does not need a power supply. except that it does not need an output inverter. Many systems used in telecommunications use 48 V DC power. some equipment accepts DC power directly and allows one or more conversion steps to be eliminated. and AC has typically been the dominant source for computers and servers. but the voltage doubler configuration also uses only half the rectifier. in the hope of reducing the likelihood of failure and the cost of equipment. There has been much experimentation with 48 V DC power for computer servers.)
. This equipment is more commonly known as a rectifier. This does cause unbalanced heating in the input rectifier stage as the full load passes through only half of it. because it has less restrictive safety regulations. the current must be greater than an equivalent 120 V or 230 V circuit. then DC to AC to power the external device. which does require AC power. DC has typically been the dominant power source for telecommunications. (Power supplies with a 115/230 V switch operate as a [citation needed] voltage doubler when in the 115 V position . High voltage DC (380 V) is finding use in some data center applications. such as being installed in conduit and junction boxes. However. and then back to DC inside the powered device. and allows for small power conductors. greater current requires larger conductors or more energy lost as heat. because the first thing they do to the AC input is rectify it.

the diesel engine can be used to directly spin the flywheel. since such short-term power events are not able to appreciably affect the rotational speed of the high-mass flywheel. Its life cycle is usually far greater than a purely electronic UPS. However. By using a transmission gearbox. unlike a battery-based UPS. flywheel-based UPS systems typically provide 10 to 20 seconds of protection before the flywheel has slowed and power output stops. A larger flywheel or multiple flywheels operating in parallel will increase the reserve running time or capacity. The flywheel also acts as a buffer against power spikes and sags. such as ball bearingreplacement.000 watts of protection. the high-mass flywheel is used in conjunction with a motor-generator system. Because the flywheels are a mechanical power source. But they do require periodic downtime for mechanical maintenance. which is usually the case for larger units. Multiple flywheels can likewise be connected in parallel through mechanical countershafts. The rotary UPS is generally reserved for applications needing more than 10. Typically. to justify the expense and benefit from the advantages rotary UPS systems bring. and are better able to provide inrush current for inductive loads such as motor startup or compressor loads. A motor driving a mechanically connected generator. Battery-based designs do not require downtime if the batteries can be hot-swapped. It is also able to tolerate short-circuit conditions up to 17 times larger than an electronic UPS. it is not necessary to use an electric motor or generator as an intermediary between it and a diesel engine designed to provide emergency power. 2. without the need for separate motors and generators for each flywheel. the rotational inertia of the flywheel can be used to directly start up a diesel engine.Rotary DRUPS (diesel rotary UPS)
Typical protection time:
20–60 seconds
Capacity expansion:
Several seconds
A rotary UPS uses the inertia of a high-mass spinning flywheel (flywheel energy storage) to provide short-term ride-through in the event of power loss. up to 30 years or more. and once running. In larger systems redundancy of the system ensures the availability of processes during this maintenance. They are normally designed to provide very high current output compared to a purely electronic UPS. It is traditionally used in conjunction with standby diesel generators. Newer rotary units use technologies such as magnetic bearings and airevacuated enclosures to increase standby efficiency and reduce maintenance to very low levels. A combined synchronous motor and generator wound in alternating slots of a single rotor and stator. permitting one device to blow a fuse and fail while other devices still continue to be powered from the rotary UPS. It can be considered to be on line since it spins continuously under normal conditions. It is also one of the oldest designs. as well as medical MRI and cath lab equipment. These units can be configured as: 1. providing backup power only for the brief period of time the engine needs to start running and stabilize its output. predating vacuum tubes and integrated circuits.
.

2 and 3 can be driven directly by an AC power source (typically when in inverter bypass). while a generator uses the flywheel to power the inverter.3. except that it uses the flywheel in place of batteries. A hybrid rotary UPS. Case No. The rectifier drives a motor to spin the flywheel. 2 and 3 can use batteries or a free-standing electrically coupled flywheel as the short-term energy source. designed similar to an online UPS. 3 the motor generator can be synchronous/synchronous or induction/synchronous.
. In case No. Case Nos. or a 6-pulse inverter. a 6-step double-conversion motor drive. 1 uses an integrated flywheel as a short-term energy source instead of batteries to allow time for external. The motor side of the unit in case Nos. electrically coupled gensets to start and be brought online.

Efficiency figures on site are often much closer to the 90% mark. This provides double protection from both a power supply failure and a UPS failure. often using different sets of criteria in order to reach these figures. or extreme heat. to a different circuit breaker). Factors such as temperature. Outdoor UPS systems can be pole. Redundancy is further enhanced by plugging each power supply into a different circuit (i. so that in the event of one power supply failing.e. one or more other power supplies are able to power the load. If the budget does not allow for two identical UPS units then it is common practice to plug one power supply into mains power and the other into the UPS. the installation will contain N+1 modules. due to varying power conditions. rain. [edit]Outdoor use When a UPS system is placed outdoors. In this way. The perfect scenario will never happen in reality. Redundant protection can be extended further yet by connecting each power supply to its own UPS. failure of one module will not impact system operation.25inch CD-ROM slot bay of a regular computer chassis. in which case the outdoor UPS system should include a battery heater mat. The first type is a miniaturized regular UPS that is made small enough to fit into a 5. [edit]Internal systems UPS systems can be designed to be placed inside a computer chassis. it should have some specific features that guarantee that it can tolerate weather with no effect on performance. humidity. multiple smaller UPS modules and batteries can be integrated together to provide redundant power protection equivalent to one very large UPS. in which case the outdoor UPS system should include a fan system or an air conditioning system. due to ongoing voltage sags from the mains and the declining efficiency of UPS batteries. and snow among others should be considered by the manufacturer when designing an outdoor UPS system. This is a critical point – each power supply must be able to power the entire server by itself. ground (pedestal). "N+1" means that if the load can be supplied by N modules. and to reach these figures companies often put their UPS in an ideal scenario. The industry norm can be argued to be anything between 93%-96% when a UPS is in full operational mode. and there are a number of reasons for this. Outdoor environment could mean extreme cold. a single huge UPS can also be a single point of failure that can disrupt many other systems. This configuration is also referred to as 1+1 or 2N redundancy. [edit]Machine
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standards
[edit]Measuring efficiency The way efficiency is measured varies massively in the UPS market.
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[edit]Multiple redundancy Many computer servers offer the option of redundant power supplies.
. Operating temperature ranges for outdoor UPS systems could be around −40 °C to +55 °C. The other type are re-engineered switching power supplies that utilize dual AC or DC power sources as inputs and have built-in switching control units.Applications
[edit]N+1 In large business environments where reliability is of great importance. To provide greater reliability. There are two types of internal UPS. so that continued operation is assured. Many UPS manufacturers claim to have the highest level of efficiency. or host mounted.

Some governors are mechanical. The standard manufacturers warranty is anything between 1–2 years and can even be limited to certain aspects of the machine. with the focus mainly on efficiency and maintenance contracts. The job of the governor is to keep the voltage and frequency constant. Many radio transmission sites will have backup diesel generators – in the case of amplitude modulation (AM) radio transmitters. companies supplying Three Phase however now offer lengthier warranties. this requires generators more than twice as big as the UPS. This problem of input frequency requirements should not be an issue through the use of a Double Conversion / online UPS. This may pose a problem where. for example. [edit]Power factor Main article: power factor A problem in the combination of a "double conversion" UPS and a generator is the voltage distortion created by the UPS. the load presented by the transmitters changes in line with the signal level. using its own internal clock source to generate the required 50 or 60 Hz supply frequency. Few companies compete on warranty.0 Hz (or 60.
. The current drawn by the UPS is non-sinusoidal. the startup surge of an elevator can cause short "blips" in the frequency of the generator or the output voltage. but its sources remain unclear because it lacks inline citations. in the event that its designers had written the microprocessor code to require exactly a50. often excluding the more expensive items such as battery replacement. with the norm closer to 2 years rather than the single year. but big and only work at full load. and present their own problems when used in tandem with generators. The input of a double conversion UPS is essentially a big rectifier. This causes the voltage from the generator also to become nonsinusoidal. thus affecting all other devices powered by the generator. with this condition not met the UPS could remain on battery power. The speed is controlled by a system called a governor. [edit]Difficulties
faced with generator use
[edit]Frequency variations The voltage and frequency of the power produced by a generator depends on the engine speed. while the load on the generator changes. The voltage distortion then can cause problems in all electrical equipment connected to the generator. Classic UPS rectifiers have a THD(i) level of around 25– 30%. This leads to the scenario where the generator is constantly trying to correct the output voltage and frequency as the load changes. (May 2012)
Warranty on uninterruptible power supplies has varied over the past couple of years. including the UPS itself. related reading or external links. To prevent voltage distortion. A UPS of this topology should be able to adapt to any input frequency. Please improve this article by introducing more precise citations. They are reliable.0 Hz) supply frequency in order to operate. often depending if a machine is Single Phase or Three Phase. There are several solutions to reduce the THD(i) in a double conversion UPS: Passive power factor correction: (Passive PFC) Classic solutions such as passive filters reduce THD(i) to 5–10% at full load. and some are electronic.[edit]Warranty This section includes a list of references. This level of "noise" is measured as a percentage of "Total Harmonic Distortion of the current" (THD(i)). Focusing on one market. being unable to reconnect the unsuitable supply voltage. It is possible for a UPS unit to be incompatible with a generator or a poor mains supply.

generates PM events or commands an ordered shut down. which for example allows one UPS to connect to five computers using serial or USB connections. and allow the computer to control the UPS. or space. A double conversion UPS with an IGBT rectifier can have a THD(i) as small as 2%. The UPS to report its status to the computer it powers via a communications link such as a serial port. a signal replication device may be used.
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However.Active power factor correction: Main article: Active power factor correction An alternative solution is an active filter. Larger systems (such as for data centers) require detailed calculation of the load. [edit]Batteries The run-time for a battery-operated UPS depends on the type and size of batteries and rate of discharge.
As Ethernet has increased in common use since the 1990s. and battery characteristics to ensure the required endurance is attained.
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the splitting is typically only one direction from UPS to the devices to provide status information. This completely eliminates the need to oversize the generator (and transformers). For traditional serial or USB control. investment cost. The basic computer-to-UPS control methods are intended for one-to-one signalling from a single source to a single target. control signals are now commonly sent between a single UPS and multiple computers using standard Ethernet data communication methods such as TCP/IP. without additional filters. a single UPS may connect to a single computer to provide status information about the UPS. A subsystem in the OS that processes the reports and generates notifications. and the efficiency of the inverter. but other manufacturers (such as APC) use proprietary protocols. The newest technology in double conversion UPS units is a rectifier that doesn't use classic rectifier components (thyristors and diodes) but high frequency components (IGBTs). In some situations it is useful for a single large UPS to be able to communicate with several protected devices. Some UPS manufacturers publish their communication protocols. which is described as Peukert's law. Manufacturers supply run-time rating in minutes for packaged UPS systems. For example. [edit]Communication
This section requires expansion. The total capacity of a lead–acid battery is a function of the rate at which it is discharged. Through the use of such a device.
Power management (PM) requires 1.
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. the Universal Serial Bus protocol is also intended to connect a single computer to multiple peripheral devices. Return control signals may only be permitted from one of the protected systems to the UPS. Ethernet or USB 2. Similarly. losses. inverter efficiency. in order for the UPS alerts to reach the target systems during a power outage. THD(i) can drop to 5% over the full power range.
Distribution of UPS status and control data requires that all intermediary devices such as Ethernet switches or serial multiplexers be powered by one or more UPS systems.
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The status and control information is typically encrypted so that for example an
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outside hacker can not gain control of the UPS and command it to shut down.

Battery strings may also be wired as series-parallel. Isolating a single cell and installing a jumper in place of it allows the one battery to be discharge-tested. With time. not just in UPS systems but also in electric vehicle applications. and monitored both individually and collectively. using intermediate sensor wires that are installed at every cell-to-cell junction. Defective batteries in one string can adversely affect the operation and lifespan of good/new batteries in other strings. and instead an [16] extendedrecalibration or rundown test that deeply discharges the battery is needed. which are at the interface between the electrodes and the electrolyte.
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such as every six months to a year. In such a situation it is also necessary to monitor current flow between parallel strings. leading to an interface charge so low that it may be insufficient to start the car. For example. as current may circulate between the strings to balance out the effects of weak cells. Other series strings wired in parallel with the degraded string will now discharge through the degraded string until their voltage matches the voltage of the degraded string. and one becomes shorted or dead:   The failed cell will reduce the maximum developed voltage for the entire series string it is within.
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It is also possible to measure the electrical characteristics of individual cells in a battery string. this initially affects only the reacting chemicals. potentially overcharging and leading to electrolyte boiling and outgassing from the remaining good cells in
. In lead acid batteries this is known as sulfation but also affects other types such as nickel cadmium batteries and lithium batteries. then during the short charging time it develops only a charge near the interface. while the rest of the battery string remains charged and available to provide protection.
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[edit]Series-parallel battery interactions Battery strings wired in series-parallel can develop unusual failure modes due to interactions between the multiple parallel strings. stronger strings can discharge through weaker strings until voltage imbalances are equalized. which consists of either combined-cell battery units (such as 12-volt lead acid batteries) or individual chemical cells wired in series. the charge stored in the chemicals at the interface.
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Consider a series-parallel battery arrangement with all good cells. If a battery has been completely discharged (e.
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Due to the interface charge. The battery voltage may rise to be close to the charger voltage so that the charging current decreases significantly. The deep discharge testing is itself damaging to batteries due to the chemicals in the discharged battery starting to crystallize into highly stable molecular shapes that will not re-dissolve when the battery is recharged. the car lights were left on overnight) and next is given a fast charge for only a few minutes.
[edit]Testing of strings of batteries/cells Multi-kilowatt commercial UPS systems with large and easily accessible battery banks are capable of isolating and testing individual cells within a battery string. or shorted cells. These issues also apply to other situations where series-parallel strings are used. After a few hours this interface charge will spread to the volume of the electrode and electrolyte.
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Therefore it is commonly recommended that rundown tests be performed infrequently. often called "interface charge". brief UPS self-test functions lasting only a few seconds may not accurately reflect the true runtime capacity of a UPS. permanently reducing charge capacity. spreads by diffusion of these chemicals throughout the volume of the active material. and this must be factored into the individual inter-cell measurements within each string.[edit]Common battery characteristics and load testing When a lead–acid battery is charged or discharged.g. for example two sets of 20 cells. dead cells with high resistance.

. [edit]Series new/old battery interactions Even just a single string of batteries wired in series can have adverse interactions if new batteries are mixed with old batteries. damaging the old cells. These parallel strings can now never be fully recharged. and may be wasted in the old cells as resistance heating. leading to a rapid rise of voltage to near the fully charged state. but before the new cells with more capacity have fully recharged. When recharged. reducing new cell capacity over several charge/discharge cycles until their capacity more closely matches the old cells in the series string.  Charging systems may attempt to gauge battery string capacity by measuring overall voltage. as the increased voltage will bleed off through the string containing the failed battery.  If lead-acid batteries are used. so slowly that the chemicals may begin to crystallize before reaching the fully charged state. The newer cells may continue to discharge through the rest of the string. due to these damaging interactions between new batteries and old batteries. resulting in the storage capacity of these cells being permanently damaged. new cells with more capacity may cause the old cells in the series string to continue to discharge beyond the safe bottom limit of the discharge window. all cells in the formerly good parallel strings will begin to sulfate due to the inability for them to be fully recharged. As a mixed string of new and old batteries is depleted. even if the damaged cell in the one degraded string is eventually discovered and replaced with a new one. and when the old batteries are exhausted the new batteries still have charge available. Older batteries tend to have reduced storage capacity. the string voltage will drop. which leads to continuous overcharging and damage to all the cells in the degraded series string containing the damaged battery. The only way to prevent these subtle series-parallel string interactions is by not using parallel strings at all and using separate charge controllers and inverters for individual series strings. Due to the overall string voltage depletion due to the dead cells. The charge controller detects the high voltage of a nearly fully charged string and reduces current flow. For such reasons. the old cells recharge more rapidly. some industrial UPS management systems recommend periodic replacement of entire battery arrays potentially using hundreds of expensive batteries.the degraded string. the charging system may detect this as a state of discharge. For cells that are supposed to operate within a specific discharge window. but due to the low voltage this energy flow may not be useful. and so will both discharge faster than new batteries and also charge to their maximum capacity more rapidly than new batteries. The new cells with more capacity now charge very slowly.within and across series and parallel strings. and will continuously attempt to charge the series-parallel strings.